Optical signal transmitter and method for controlling polarization multiplexed optical signal

ABSTRACT

An optical signal transmitter includes: first outer modulator to generate first modulated optical signal, the first outer modulator including a pair of optical paths and a first phase shifter to give phase difference to the pair of optical paths; second outer modulator to generate second modulated optical signal, the second outer modulator including a pair of optical paths and a second phase shifter to give phase difference to the pair of optical paths; combiner to generate polarization multiplexed optical signal by combining the first and second modulated optical signals; phase controller to control the phase difference by the first phase shifter to A−Δφ and control the phase difference by the second phase shifter to A+Δφ; and power controller to control at least one of the first and second outer modulators based on AC component of the polarization multiplexed optical signal.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is based upon and claims the benefit of priority of theprior Japanese Patent Application No. 2009-175212, filed on Jul. 28,2009, the entire contents of which are incorporated herein by reference.

FIELD

The present invention relates an optical signal transmitter and a methodfor controlling polarization multiplexed optical signal. The inventionmay be applied to, for example, an optical signal transmitter used in apolarization multiplexed transmission system.

BACKGROUND

The needs for super high speed transmission systems with a speed of morethan 40 Gbit/s (for example, 100 Gbit/s) have been increasing rapidly.For this reason, development is under way for practical realization ofan optical transmission system that adopts a multilevel modulationscheme applied to a radio system (for example, the QPSK modulation usingfour-level phase modulation). However, as the transmission-signal speedbecomes higher, solving problems related to the feasibility of theelectric signal circuit and problems related to the degradation of theoptical transmission signal (transmission-signal spectrum degradationdue to an optical filter and signal degradation due to chromaticdispersion and accumulation of optical noises) becomes more difficult.

As one of techniques for solving these problems, optical polarizationmultiplexing has attracted attention. A polarization multiplexed opticalsignal is generated by, for example, an optical signal transmitterillustrated in FIG. 1A. The optical signal transmitter has a lightsource (LD), a pair of modulators, and a polarization beam combiner(PBC). Continuous wave light output from the light source is spilt andguided to the pair of modulators. The pair of modulators modulate thecontinuous wave light respectively with corresponding data signal, andgenerate a pair of modulated optical signals. The polarization beamcombiner generates a polarization multiplexed optical signal illustratedin FIG. 1B by combining the pair of modulated optical signals. In otherwords, in the polarization multiplexing, two data streams aretransmitted using two polarized waves (X polarization and Ypolarization) that have the same wavelength and are orthogonal to eachother.

Accordingly, in the polarization multiplexing, the data speed becomeshalf, realizing the improvement of the characteristics of theelectric-signal generation circuit and reduces the cost, size and powerconsumption of the circuit. In addition, the characteristics of theoptical transmission system as a whole is improved, as influences fromquality-degradation factors such as dispersion on the opticaltransmission path are reduced. Japanese Laid-open Patent Publication No.62-24731, Japanese Laid-open Patent Publication No. 2002-344426,Japanese Laid-open Patent Publication No. 2008-172799 describe atransmission system using the polarization multiplexing. In addition, asrelated arts, configurations described in Japanese Laid-open PatentPublication No. 2009-63835 and Japanese Laid-open Patent Publication No.2008-172714 have been proposed.

However, in an optical signal transmitter that generates a polarizationmultiplexed optical signal, a modulator is provided for eachpolarization as illustrated in FIG. 1A. For this reason, differences inthe optical power may be generated between the polarization in thepolarization multiplexed output signal, due to factors such asmanufacturing variability of characteristics between the modulators (forexample, loss of the LN modulator) and characteristics of the opticalsplitter and/or the optical combiner.

SUMMARY

According to an aspect of an invention, an optical signal transmitterincludes a first outer modulator to generate a first modulated opticalsignal, the first outer modulator including a pair of optical paths anda first phase shifter to give a phase difference to the pair of opticalpaths; a second outer modulator to generate a second modulated opticalsignal, the second outer modulator including a pair of optical paths anda second phase shifter to give a phase difference to the pair of opticalpaths; a combiner to generate a polarization multiplexed optical signalby combining the first and second modulated optical signals; a phasecontroller to control the phase difference by the first phase shifter toA−Δφ and control the phase difference by the second phase shifter toA+Δφ when the first and second outer modulators are driven by first andsecond control data signals respectively; and a power controller tocontrol at least one of the first and second outer modulators based onan AC component of the polarization multiplexed optical signal when thefirst and second outer modulators are driven by the first and secondcontrol data signals respectively. The data pattern of the first controldata signal is same as the second control data signal. The data patternof the first control data signal may be reversed pattern of the secondcontrol data signal.

According to another aspect of an invention, an optical signaltransmitter includes: a first light source; a first outer modulator togenerate a first modulated optical signal by modulating an opticalsignal generated by the first light source, the first outer modulatorincluding a pair of optical paths and a first phase shifter to give aphase difference to the pair of optical paths; a second light source; asecond outer modulator to generate a second modulated optical signal bymodulating an optical signal generated by the second light source, thesecond outer modulator including a pair of optical paths and a secondphase shifter to give a phase difference to the pair of optical paths; acombiner to generate a polarization multiplexed optical signal bycombining the first and second modulated optical signals; a phasecontroller to control the phase difference by the first phase shifter toA−Δφ and control the phase difference by the second phase shifter toA+Δφ when the first and second outer modulators are driven by first andsecond control data signals respectively; and a power controller tocontrol at least one of the first and second light sources based on anAC component of the polarization multiplexed optical signal when thefirst and second outer modulators are driven by the first and secondcontrol data signals respectively. The data pattern of the first controldata signal is same as the second control data signal. The data patternof the first control data signal may be reversed pattern of the secondcontrol data signal.

According to another aspect of an invention, a method for controlling apolarization multiplexed optical signal is used in an optical signaltransmitter including a first outer modulator having a pair of opticalpaths and a first phase shifter to give a phase difference to the pairof optical paths to generate a first modulated optical signal; a secondouter modulator having a pair of optical paths and a second phaseshifter to give a phase difference to the pair of optical paths togenerate a second modulated optical signal; and a combiner to generatethe polarization multiplexed optical signal by combining the first andsecond modulated optical signals. The method includes: controlling thephase difference by the first phase shifter to A−Δφ; controlling thephase difference by the second phase shifter to A+Δφ; generating firstand second control data signals as drive signals of the first and secondouter modulators, respectively; and controlling at least one of thefirst and second outer modulators based on an AC component of thepolarization multiplexed optical signal generated when the first andsecond outer modulators are driven by the first and second control datasignals, respectively. The data pattern of the first control data signalis same as the second control data signal. The data pattern of the firstcontrol data signal may be reversed pattern of the second control datasignal.

The object and advantages of the invention will be realized and attainedby means of the elements and combinations particularly pointed out inthe claims.

It is to be understood that both the foregoing general description andthe following detailed description are exemplary and explanatory and arenot restrictive of the invention, as claimed.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1A and FIG. 1B are diagrams illustrating polarization multiplexing.

FIG. 2 is a diagram illustrating the configuration of an optical signaltransmitter according to an embodiment.

FIG. 3 is a diagram illustrating a first embodiment of an optical signaltransmitter.

FIG. 4 is a diagram illustrating the operation of an LN modulator.

FIG. 5 is a simulation result (Δφ=90 degrees) illustrating the relationbetween the power difference and the monitor signal in the firstembodiment.

FIG. 6 is a simulation result (Δφ=45 degrees) illustrating the relationbetween the power difference and the monitor signal in the firstembodiment.

FIG. 7 is a simulation result (random data input) illustrating therelation between the power difference and the monitor signal in thefirst embodiment.

FIG. 8 is a diagram illustrating a second embodiment of the opticalsignal transmitter.

FIG. 9 is a simulation result (Δφ=90 degrees) illustrating the relationbetween the power difference and the monitor signal in the secondembodiment.

FIG. 10 is a simulation result (Δφ=45 degrees) illustrating the relationbetween the power difference and the monitor signal in the secondembodiment.

FIG. 11 is a diagram illustrating a third embodiment of the opticalsignal transmitter.

FIG. 12 is a diagram illustrating a fourth embodiment of the opticalsignal transmitter.

FIG. 13 is a diagram illustrating the input and output of the LNmodulator in the case where a pair of drive data are in reverse phasewith each other.

FIG. 14 through FIG. 16 are diagrams illustrating a method ofcontrolling the phase shifter.

FIG. 17 is a diagram illustrating the configuration of an optical signaltransmitter that performs online control.

FIG. 18 is a flowchart illustrating the operation of the controllerillustrated in FIG. 17.

FIG. 19 is a diagram illustrating the configuration of an optical signaltransmitter according to another embodiment.

FIG. 20 is a flowchart illustrating the operation of the controllerillustrated in FIG. 19.

DESCRIPTION OF EMBODIMENTS

FIG. 2 illustrates the configuration of an optical signal transmitteraccording to an embodiment. The optical signal transmitter according tothe embodiment transmits a polarization multiplexed optical signalobtained by combining first and second modulated optical signals. Thepolarization multiplexed optical signal carries data to a receivingstation using X polarization and Y polarization that are orthogonal toeach other. Here, if the power of the X polarization and that of the Ypolarization are different from each other, the characteristics of thepolarization multiplexed optical signal degrade. Therefore, in theoptical signal transmitter according to the embodiment, control isperformed to reduce (desirably, to minimize) the power differencebetween the X polarization and the Y polarization.

The light source (LD) 1 is, for example, a laser diode, and generates anoptical signal having a certain frequency. The optical signal is, forexample, a continuous wave (CW). The optical signal is split by, forexample, an optical splitter and guided to modulators 10 and 20.

The modulators 10 and 20 are, in this example, modulators (for example,Mach-Zehnder LN modulators) by which the power of the output lightperiodically changes according to the drive voltage. The modulator 10has a pair of optical paths and a phase shifter 11 that gives a phasedifference to the pair of optical paths. The modulator 10 generates amodulated optical signal X by modulating an input optical signalaccording to data X. In the same manner, the modulator 20 has a pair ofoptical paths and a phase shifter 21 that gives a phase difference tothe pair of optical paths. The modulator 20 generates a modulatedoptical signal Y by modulating an input optical signal according to dataY.

A driver 12 generates a drive voltage signal representing the data X andgives the signal to the modulator 10 A driver 22 generates a drivevoltage signal representing the data Y and gives the signal to themodulator 20. Meanwhile, in order to control the operating point (thatis, the bias) of the LN modulators, the modulators 10 and 20respectively has a bias circuit that is not illustrated in the drawing.The bias circuit is, for example, an ABC (auto bias control) circuit.For example, the ABC circuit applies a low-frequency voltage signal tothe corresponding LN modulator, and based on the low-frequency componentcontained in the output light from the modulators 10 and 20, adjusts theoperating point (that is, the DC bias voltage) of the corresponding LNmodulator.

Note that the LN modulator is described as an example of the opticalmodulator herein, this is not a limitation. The optical modulator is notlimited to the LN modulator, and may be a modulator usingelectro-optical materials, for example, a modulator includingsemiconductor materials such as InP.

Optical attenuators 13 and 23 adjust the power of the modulated opticalsignals X and Y, respectively. The optical attenuators 13 and 23 are notindispensable constituent elements. In addition, the optical attenuators13 and 23 may be provided on the input side of the modulators 10 and 20,or may be provided within the modulators 10 and 20, or may be providedon the output side of the modulators 10 and 20.

In the configuration described above, an outer modulator that generatesthe modulated optical signal X may be configured to include themodulator 10, the driver 12, the bias circuit not illustrated in thedrawing, and the optical attenuator 13. Similarly, an outer modulatorthat generates the modulated optical signal Y may be configured toinclude the modulator 20, the driver 22, the bias circuit notillustrated in the drawing, and the optical attenuator 23.

A polarization beam combiner (PBC) 31 generates a polarizationmultiplexed optical signal by polarization multiplexing the modulatedoptical signal X and the modulated optical signal Y. Here, in thepolarization multiplexing, as illustrated in FIG. 1B, the X polarizationand the Y polarization that are orthogonal to each other are used. Themodulated optical signal X is propagated using the X polarization, andthe modulated optical signal Y is propagated using the Y polarization.

In the configuration described above, when the optical transmittertransmits data, the phase shifters 11 and 21 are controlled so as togenerate a phase determined according to the modulation scheme. Forexample, in QPSK (including DQPSK), the phase φ of both the phaseshifters 11 and 21 is controlled to π/2. Meanwhile, a data generator 40generates transmission data X and Y. When the transmission data X and Yare given to the modulators 10 and 20 as the data X and Y respectively,the modulated optical signals X and Y are generated, and polarizationmultiplexed optical signal that carries the modulated optical signals Xand Y are output.

The optical signal transmitter according to the embodiment has thefollowing control system for controlling the power difference betweenthe X polarization and the Y polarization to zero or approximately zero.A photo detector (PD) 51 converts a polarization multiplexed opticalsignal split by the optical splitter into an electric signal. An ACcomponent extracting device 52 extracts the AC component from theelectric signal obtained by the photo detector 51 (or, removes the DCcomponent). A low pass filter 53 removes the symbol frequency componentof the data X and Y from the electric signal. An AC component powerdetector 54 detects the AC component power of the electric signal forwhich filtering has been performed by the low pass filter 53, andoutputs the detection result as a monitor signal. A controller 55controls at least one of the outer modulators (the modulator, driver,bias circuit, optical attenuator) according to the monitor signalrepresenting the AC component power of the polarization multiplexedoptical signal. Note that the low pass filter 53 is provided between thephoto detector 51 and the AC component power detector 54 in the exampleillustrated in FIG. 2, however, an electric circuit having asufficiently low band with respect to the modulation speed may beprovided instead of the low pass filter 53.

When adjusting the power balance of the X polarization and the Ypolarization, a control data generator 41 generates control data X andY. The data pattern of the control data X is the same as the datapattern of the control data Y. The generated control data X and Y aregiven to the modulators 10 and 20 through the drivers 12 and 22 as thedata X and Y, respectively. In other words, while the power balance isadjusted, the modulators 10 and 20 are driven by the same control data.

In addition, when adjusting the power balance between the X polarizationand the Y polarization, a phase controller 56 controls the phases of thephase shifter 11 and 21 to “A−Δφ” and “A+Δφ”, respectively. In otherwords, the phase difference “A−Δφ” is given to the pair of optical pathsof the modulator 10, and the phase difference “A+Δφ” is given to thepair of optical paths of the modulator 20. Here, “A” is the optimalphase at the time of data transmission, which is, for example, π/2 forQPSK or DQPSK. Meanwhile, “Δφ” is an arbitrary phase other than zero.The phases of the phase shifters 11 and 21 are set so that they areshifted to opposite directions to each other with respect to the optimalphase by the same amount. The phase controller 56 may be provided eitherin a controller 55 or outside the controller 55.

In the setting described above, the controller 55 generates one or moreof control signals C1-C3 so as to reduce (preferably, to minimize) theAC component power of the polarization multiplexed optical signal. To“reduce the AC component power” may be an operation to control the ACcomponent power below a specified threshold value. The control signal C1controls the amplitude of the drive voltage generated by the drivers 12and 22. The output power of the modulators 10 and 20 depends on thedrive voltage. The output power is high with a large drive amplitude,and the output power is low with a small drive amplitude. The controlsignal C2 controls the bias of the modulators 10 and 20. The outputpower of the modulators 10 and 20 depends on the DC bias voltagecontrolling the bias point. Generally, the output power becomes maximumwhen the bias is controlled to the optimal point. The control signal C3controls the attenuation amount of the optical attenuators 13 and 23.

In the setting described above, when the AC component power of thepolarization multiplexed optical signal is minimized, while the detailsare to be described later, the power difference between the Xpolarization and the Y polarization contained in the polarizationmultiplexed optical signal is minimized. Therefore, by appropriatelygenerating one or more of the control signal C1 to C3, the powerdifference between the X polarization and the Y polarization isminimized, improving the characteristics of the polarization multiplexedoptical signal.

As described above, the optical signal transmitter according to theembodiment has a feedback system that controls the outer modulator usingthe AC component power of the polarization multiplexed optical signal,and the feedback system adjusts the power balance between the Xpolarization and the Y polarization. In other words, without monitoringthe power of the modulated optical signals X and Y separately, the powerdifference between the polarizations are adjusted by monitoring thepolarization multiplexed optical signal output from the opticaltransmitter. This makes it possible to optimize the power balancebetween the X and Y polarizations based on the actually-outputpolarization multiplexed optical signal, without depending on thevariation in the characteristics of a photo detector that monitors thepower of the modulated optical signals X and Y separately, or in thecharacteristics of the polarization beam combiner.

However, in the control method according to the embodiment, the controldata are provided as the data X and Y, and the phases of the phaseshifters 11 and 21 are respectively out of the optimal phase. Therefore,the control method is performed when the optical signal transmitter isoffline (at the time of system startup, wavelength switching, etc).Then, during data transmission, the optical signal transmitter operatesin the state (the drive voltage, bias or attenuation amount) obtained bythe feedback control described above. At this time, the phases of thephase shifters 11 and 21 are respectively controlled to the optimalphase.

FIG. 3 is a diagram illustrating a first embodiment of the opticalsignal transmitter. In this example, data are transmitted in NRZ-DQPSKscheme. The modulation scheme is not limited to QPSK/DQPSK, and may beanother DPSK modulation scheme or another multilevel modulation scheme.For example, as disclosed in United States Patent Application No.2006/0127102, the optical transmitter may be one having an opticalmodulator that changes the optical phase as vector by performing afiltering process for a data signal.

In FIG. 3, the modulator 10 is a Mach-Zehnder DQPSK optical modulator,and in this example, has LN modulators (inner modulators) 10 a and 10 b,and the phase shifter 11. The LN modulators 10 a and 10 b are, in thisexample, Mach-Zehnder interferometers. The LN modulator 10 a is providedin one of a pair of optical paths (I arm and Q arm), and the LNmodulator 10 b is provided in another of the pair of optical paths. Thephase shifter 11 gives the phase difference π/2 between the I arm and Qarm. The phase shifter 11 may be realized by a material whose opticalpath length (or refractive index) changes according to the voltage ortemperature, for example. However, the phase of the phase shifter 11 isadjusted to π/2−Δφ when the power balance between the polarizations isadjusted.

A driver 12 drives the LN modulators 10 a and 10 b using drive signalsData I and Data Q. Here, drive signals Data I and Data Q are, forexample, generated by encoding data X by a DQPSK encoder. In addition,the driver 12 has an amplifier and is capable of controlling theamplitude of the drive signals Data I and Data Q. The driver 12illustrated in FIG. 3 is configured to output a differential signal,however, the driver 12 may provide single output.

FIG. 4 is a diagram illustrating the operation of the LN modulator. Thepower of the output light of the LN modulator changes periodically withrespect to the drive voltage. Here, the drive amplitude is “2Vπ”, where“Vπ” is a half-wavelength voltage, which is a voltage for the power ofthe output light of the LN modulator to change from a local minimum tothe local maximum. Therefore, in FIG. 3, when the amplitude of the drivesignal Data I is reduced, the amplitude of the output optical signal ofthe LN modulator 10 a becomes small, and the average power of the outputlight of the LN modulator 10 a decreases. Similarly, when the amplitudeof the drive signal Data Q is reduced, the average power of the outputlight of the LN modulator 10 b decreases. The amplitude of the drivesignals Data I and Data Q is controlled by the adjustment of the gain ofthe amplifier provide in the driver 12. If an amplifier with fixed gainis used, a similar effect may be obtained by adjusting the input signalamplitude of the amplifier. The powers of the output lights of the LNmodulators 10 a and 10 b are controlled to be the same as each other.

Meanwhile, when the operating point of the modulator is shifted byadjusting the DC bias voltage applied to the LN modulators 10 a and 10b, the average powers of the output lights of the LN modulators 10 a and10 b change. In other words, for example in FIG. 4, when the DC voltageof the drive signal is adjusted, the corresponding output light signalchanges, and the average power of the output light changes. Therefore,the power of the output light may be controlled by adjusting the DC biasvoltage applied to the LN modulators 10 a and 10 b.

The configuration and operation of the modulator 20 is basically thesame as those of the modulator 10. The modulator 20 has LN modulators 20a and 20 b, and the phase shifter 21. The phase shifter 21 gives thephase difference π/2 between the I arm and Q arm in the same manner asthe phase shifter 11. However, the phase of the phase shifter 21 isadjusted to π/2+Δφ when the power balance between the polarizations isadjusted.

The optical signal transmitter configured as described above transmits apair of data X and Y using a polarization multiplexed optical signal.The modulator 10 is driven according to the data X, and a modulatedoptical signal X is generated. In the same manner, the modulator 20 isdriven according to the data Y, and a modulated optical signal Y isgenerated. The modulated optical signals X and Y are guided to thepolarization beam combiner 31. Then, the polarization beam combiner 31generates a polarization multiplexed optical signal by polarizationmultiplexing of the modulated optical signals X and Y. The polarizationmultiplexed optical signal is transmitted through an optical fibertransmission path.

The optical signal transmitter illustrated in FIG. 3 has the controlsystem (the photo detector 51, the AC component extracting device 52,the low pass filter 53, the AC component power detector 54, thecontroller 55) described with reference to FIG. 2. The photo detector 51is, for example a photo diode, which converts the polarizationmultiplexed optical signal split by an optical splitter into an electricsignal.

The low pass filter 53 removes the symbol frequency component of thedata X and Y from the electric signal. For example, when the symbolfrequency of the data X and Y is 20 Gsymbol/s, the low pass filter 53removes at least the 20 GHz component. The AC component extractingdevice 52 removes the DC component from the electric signal, andextracts the AC component. The AC component extracting device 52 isrealized, for example, by a capacitor that removes the DC component.Note that there is no particular limitation for the order in which thelow pass filter 53 and the AC component extracting device 52. The ACcomponent extracting device 52 may be provided on the input side of thelow pass filter 53, or the AC component extracting device 52 may beprovided on the output side of the low pass filter 53.

The AC component power detector 54 detects the power of the electricsignal obtained as described above, and outputs the detection result asa monitor signal. Therefore, the monitor signal represents the ACcomponent power of the electric signal that corresponds to thepolarization multiplexed optical signal. The AC component power detector54 may be realized by an analog circuit, or by a processor that performsdigital operations. When the AC component power detector 54 is realizedby a processor, the electric signal may be converted into digital databy an A/D converter not illustrated in the drawing and input to the ACcomponent power detector 54.

The controller 55 generates one or more of control signals C1-C3 forminimizing the monitor signal. The control signal C1 is given to thedrivers 12 and/or 22. That is, the amplitude of the drive signal thatdrives the modulator 10 and/or the amplitude of the drive signal thatdrives the modulator 20 is controlled by the control signal C1. Thecontrol signal C2 is given to the bias circuit that controls the bias ofthe modulators 10 and/or 20. That is, the bias point of the modulator 10and/or the bias point of the modulator 20 is controlled by the controlsignal C2. The control signal C3 is given to the optical attenuators 13and/or 23. That is, the attenuation amount of the optical attenuator 13and/or the attenuation amount of the optical attenuator 23 is controlledby the control signal C3.

In the optical signal transmitter according to the first embodiment,feedback control is performed using one or more of the control signalsC1-C3 described above. In other words, in the first embodiment, thepower difference between the X polarization and the Y polarization iscontrolled by controlling one or more of the amplitude of the drivevoltage, the bias of the modulator, and the attenuation amount of theoptical attenuator.

In the optical signal transmitter configured as described above, whenthe power balance of the X polarization and the Y polarization isadjusted, the following state is set as described with reference to FIG.2.

(1) Control data X and Y having the same data pattern as each other aregiven as data X and Y. It is preferable that the data X and Y are inputto the modulators 10 and 20 in synchronization with each other and atthe same timing. However, the input timing of the data X and Y maycontain an error that is shorter than one symbol time.(2) The phases of the phase shifters 11 and 21 are set to “A−Δφ” and“A+Δφ”, respectively. In this example, A=π/2 as the modulation scheme isDQPSK.

FIG. 5 is a simulation result that illustrates the power differencebetween the polarizations and the monitor signal. In this simulation,the cases where the power difference between the X polarization and theY polarization is zero, 3 dB and 6 dB are compared. In addition, Δφ=90degrees. In other words, the phase difference φ between the LNmodulators 10 a and 10 b is zero in the modulator 10, and the phasedifference φ between the LN modulators 20 a and 20 b is 180 degrees inthe modulator 20. Meanwhile, “X POLARIZATION”, “Y POLARIZATION” and“POLARIZATION MULTIPLEXED OPTICAL SIGNAL” in FIG. 5 represent theoptical power. In addition, “MONITOR SIGNAL” in FIG. 5 represents theinput signal of the AC component power detector 54.

The control data X and Y given to the modulator 10 and 20 have, asdescribed above, the same data pattern as each other. For this reason,when the phase difference φ in the modulator 10 is zero and the phasedifference φ in the modulator 20 is 180 degrees, the data pattern of themodulated optical signal X (hereinafter, X polarization data) outputfrom the modulator 10 and the data pattern of the modulated opticalsignal Y (hereinafter, Y polarization data) output from the modulator 20are in the opposite phases from each other. When the X polarization datais “1 (light emitting status)”, the Y polarization data is “0 (no lightemission status)”. When the X polarization data is “0”, the Ypolarization data is “1”.

Accordingly, when the power difference between the X polarization andthe Y polarization is small, the optical levels of the respectivesymbols in the polarization multiplexed optical signal output from thepolarization beam combiner 31 become approximately the same. That is,the power variation (that is, the AC component power) of thepolarization multiplexed optical signal becomes small. In contrast, whenthe power difference between the X polarization and the Y polarizationbecomes large, an optical power difference is generated between thesymbols in the polarization multiplexed optical signal. That is, thepower variation of the polarization multiplexed optical signal alsobecomes large. Thus, when the power difference between the Xpolarization and the Y polarization is small, the monitor signal thatrepresents the AC component power of the polarization multiplexedoptical signal also becomes small, and when the power difference betweenthem becomes large, the monitor signal also becomes large. Meanwhile,the monitor signal output from the AC component power detector 54 is theaverage value or the integration value of “MONITOR SIGNAL” in FIG. 5.

As described above, in the optical signal transmitter according to theembodiment, when the power difference between the X polarization and theY polarization is small, the monitor signal also becomes small.Therefore, when the controller 55 minimizes the monitor signal by thefeedback control, the power difference between the X polarization andthe Y polarization is also minimized. In order to minimize the monitorsignal, at least one of the drive voltage of the drivers 12 and/or 22,the bias of the modulators 10 and/or 20, and the attenuation amount ofthe optical attenuators 13 and/or 23 is controlled, as described above.

FIG. 6 is a simulation result in the case where Δφ=45 degrees. The phasedifference φ in the modulator 10 is 45 degrees, and the phase differenceφ in the modulator 20 is 135 degrees.

In the case where the phase difference in the modulators 10 and 20 areset in this way, the monitor signal also becomes small when the powerdifference between the X polarization and the Y polarization is small.Therefore, there is no particular limitation on Δφ as long as it is avalue other than zero. However, the sensitivity of the monitor signal isthe best when Δφ=90 degrees in QPSK/DQPSK.

FIG. 7 is a simulation result when the data X and Y are not the same aseach other (that is, a random pattern), where Δφ=90. In this case, evenwhen the power difference between the X polarization and the Ypolarization is zero, the power variation of the polarizationmultiplexed optical signal is large. That is, the AC component power ofthe monitor signal is hardly dependent on the power difference betweenthe X polarization and the Y polarization. Therefore, when the data Xand Y are different from each other, it is difficult to control thepower difference between the X polarization and the Y polarization usingthe AC component power of the polarization multiplexed optical signal.

Here, the control of the phases of the phase shifters 11 and 21 to“A−Δφ” and “A+Δφ” respectively and the same control data being given tothe modulators 10 and 20 are described. In the following description, itis assumed that the modulation scheme is DQPSK, and A=π/2. It is alsoassumed that when the power balance is adjusted, Δφ=90. In other words,when the power balance is adjusted, the phase φ of the phase shifter 11of the modulator 10 is zero, and the phase φ of the phase shifter 21 ofthe modulator 20 is 180 degrees. Furthermore, it is assumed that theoptical devices in the optical signal transmitter have idealcharacteristics.

In DQPSK, 2-bit data are transmitted with every one symbol. Then, it isassumed that at the time of data transmission (that is, φ=π/2), eachsymbol is transmitted in the following state.

symbol (1,1): the phase of the carrier=π/4,optical power=√2symbol (0,1): the phase of the carrier=3π/4,optical power=√2symbol (0,0): the phase of the carrier=5π/4,optical power=√2symbol (1,0): the phase of the carrier=7π/4,optical power=√2

In this case, while the power balance is being adjusted, the state ofthe modulated optical signal X obtained by the modulator 10 (φ=0)becomes as follows.

symbol (1,1): the phase of the carrier=0, optical power=2symbol (0,1): optical power=0symbol (0,0): the phase of the carrier=π, optical power=2symbol (1,0): optical power=0

At this time, the state of the modulated optical signal Y obtained bythe modulator 20 (φ=180) becomes as follows.

symbol (1,1): optical power=0symbol (0,1): the phase of the carrier=0, optical power=2symbol (0,0): optical power=0symbol (1,0): the phase of the carrier=π, optical power=2

Here, in order to make the explanation simple, it is assumed that thepower of the polarization multiplexed optical signal is the sum of theoptical powers of the modulated optical signals X and Y. In addition,while the power balance is being adjusted, the same control data aregiven to the modulators 10 and 20. Then, the power of the polarizationmultiplexed optical signal becomes as follows.

symbol (1,1): optical power=2 (=2+0)symbol (0,1): optical power=2 (=0+2)symbol (0,0): optical power=2 (=2+0)symbol (1,0): optical power=2 (=0+2)

Thus, in the optical signal transmitter according to the embodiment,when the phases of the phase shifters 11 and 21 are controlled to “A−Δφ”and “A+Δφ” respectively and the same control data are given to themodulators 10 and 20, the variation of the power of the polarizationmultiplexed optical signal is small (ideally zero). That is, the ACcomponent power of the polarization multiplexed optical signal becomessmall.

At this time, it is assumed that, for example, the power of themodulated optical signal Y obtained by the modulator 20 (φ=180)decreases by 3 dB. In this case, the state of the modulated opticalsignal Y becomes as follows.

symbol (1,1): optical power=0symbol (0,1): the phase of the carrier=0, optical power=1symbol (0,0): optical power=0symbol (1,0): the phase of the carrier=π, optical power=1

Then, the power of the polarization multiplexed optical signal becomesas follows.

symbol (1,1): optical power=2 (=2+0)symbol (0,1): optical power=1 (=0+1)symbol (0,0): optical power=2 (=2+0)symbol (1,0): optical power=1 (=0+1)

Thus, the power difference between the optical modulation X and Y (thatis, the power difference between the X polarization and the Ypolarization) occurs, the variation of the power of the polarizationmultiplexed optical signal becomes large. Therefore, the optical signaltransmitter according to the embodiment controls at least one of themodulators 10 and 20 so as to minimize the monitor signal thatrepresents the AC component power of the polarization multiplexedoptical signal. Accordingly, the power difference between thepolarizations is minimized.

Meanwhile, in the optical signal transmitter according to theembodiment, since feedback control is performed using the optical signalafter polarization multiplexing, the optical power difference betweenthe polarizations generated between the light source 1 and thepolarization beam combiner 31 is controlled. Therefore, the powerdifference (control error) between the polarizations of several dB thatcould occur due to the characteristics of the photo detector provided inthe respective modulators 10 and 20 or the characteristics of thepolarization beam combiner 31 is compensated for. In addition, thecircuit elements from the photo detector 51 to the AC component powerdetector 54 may be shared with a part of the circuit that controls thebias of the modulators 10 and 20, and/or a part of the circuit thatcontrols the phase shifters 11 and 21. In this configuration, the sizereduction or simplification of the optical signal transmitter isrealized.

FIG. 8 is a diagram illustrating a second embodiment of the opticalsignal transmitter. The optical signal transmitter according to thesecond embodiment has an RZ optical modulator on the input side oroutput side of the DQPSK optical modulator. In the example illustratedin FIG. 8, RZ optical modulators 61 and 71 are provided on the outputside of the modulators (DQPSK optical modulators) 10 and 20,respectively. Therefore, in the second embodiment, data are transmittedin RZ-DQPSK modulation scheme. Note that the modulators 10 and 20operate as phase modulators, and the RZ optical modulators 61 and 71operate as intensity modulators.

The RZ optical modulators 61 and 71 are, for example, Mach-Zehnder LNmodulators, and perform RZ modulation according to a drive signalgenerated by driver circuits 62 and 72, respectively. Here, drivercircuits 62 and 72 generate a drive signal synchronized with the symbolclock. The drive signal is, while there is no particular limitation, forexample, a sine wave of the same frequency as the clock. The amplitudeof the drive signal is, for example, Vπ.

The RZ optical modulators 61 and 71 has, in the same manner as themodulators 10 and 20, a bias circuit (ABC circuit) not illustrated inthe drawing, in order to control the operating point of the LNmodulator. The power of the output light may be controlled by adjustingthe DC bias voltage applied to the RZ optical modulators 61 and 71.

The optical attenuators 13 and 23 may be omitted. In addition, theoptical attenuators 13 and 23 may be provided on the input side of themodulators 10 and 20, or may be provided between the modulators 10 and20 and the RZ optical modulators 61 and 71, respectively.

In the optical transmitter configured as described above, theconfiguration and the operation of the control system controlling thepower difference between the polarizations is basically the same asthose in the first embodiment. That is, the controller 55 generates acontrol signal for minimizing the monitor signal corresponding to thepolarization multiplexed optical signal. However, the controller 55 inthe second embodiment generates a control signal C4. The control signalC4 controls the bias of the RZ optical modulators 61 and 71. In otherwords, the control signal C4 adjusts the output optical power of the RZoptical modulator 61 and/or the output optical power of the RZ opticalmodulator 71. Therefore, the power difference between the X polarizationand the Y polarization is optimized by feedback control using thecontrol signal C4. Note that in the configuration of the secondembodiment, the power difference may be controlled also using thecontrol signals C1-C3.

FIG. 9 and FIG. 10 are simulation results illustrating the relationbetween the power difference and the monitor signal in the secondembodiment. FIG. 9 and FIG. 10 represent the simulation results in thecases where Δφ=90 degrees and Δφ=45 degrees, respectively.

In the second embodiment (RZ-DQPSK), similarly to the first embodiment(NRZ-DQPSK), when the power difference between the X polarization andthe Y polarization becomes small, the monitor signal also becomes small.Therefore, the power difference between the X polarization and the Ypolarization may be reduced when feedback control is performed so as tominimize the monitor signal. Note that there is no limitation on thevalue of Δφ (≠0) also in the second embodiment. In addition, accordingto RZ-DQPSK, when the power difference between the X polarization andthe Y polarization becomes zero, the monitor signal takes a value closeto zero. Therefore, the sensitivity of the adjustment with respect tothe monitor signal becomes higher in RZ-DQPSK.

FIG. 11 is a diagram illustrating a third embodiment of the opticalsignal transmitter. The optical signal transmitter according to thethird embodiment has the configuration to detect the minimum point ofthe AC component power using synchronized detection. FIG. 11 illustratesthe configuration in which the attenuation amount of the opticalattenuators 13 and 23 is controlled using the control signal C3.

A low-frequency signal generator 81 generates a low-frequency signal.The frequency of the low-frequency signal is sufficiently low withrespect to the symbol frequency. The frequency of the low-frequencysignal is, for example, several hundreds Hz-several MHz. A superimposer(adder) 82 superimposes (adds) the low-frequency signal on the controlsignal C3 generated by the controller 55. The control signal C3 on whichthe low-frequency signal has been superimposed is then given to theoptical attenuators 13 and 23. Accordingly, the power of the modulatedoptical signals X and Y oscillates at the frequency of the low-frequencysignal, and the power of the polarization multiplexed optical signalalso oscillates at the frequency of the low-frequency signal. Therefore,the monitor signal output from the AC component power detection unit 54also oscillates at the frequency of the low-frequency signal.

A low-frequency signal is given from the low-frequency signal generator81 to the controller 55. The controller 55 uses the low-frequency signalto perform synchronized detection of the monitor signal. That is, thecontroller 55 detects the monitor signal by synchronized detection. Thefeedback control by the controller 55 for minimizing the detectedmonitor signal is the same as that in the first embodiment. Note thatthe synchronized detection may be applied to a configuration in whichthe control signals C1 and C2 are used, as well as to RZ-DQPSKillustrated in FIG. 8.

FIG. 12 is a diagram illustrating a fourth embodiment of the opticalsignal transmitter. In the optical signal transmitter in the fourthembodiment, light sources 2 and 3 are provided for the modulators 10 and20, respectively. The modulator 10 generates the modulated opticalsignal X using the output of the light source 2, and the modulator 20generates the modulated optical signal Y using the output of the lightsource 3.

In the optical signal transmitter configured as described above, thecontroller 55 generates a control signal C5. The control signal C5 isgiven to the light sources 2 and/or 3. The light sources 2 and 3controls the output optical power according to the control signal C5. Atthis time, the controller 55 generates the control signal C5 whileperforming feedback control so as to minimize the monitor signal.Accordingly, the optical powers of the X polarization and the Ypolarization of the polarization multiplexed optical signal may becomeapproximately the same as each other. Here, the optical signaltransmitter of the fourth embodiment may generate the control signalsC1-C3. Thus, in the fourth embodiment, the power difference between thepolarizations is controlled by controlling one or more of the amplitudeof the drive voltage, bias of the modulator, the attenuation amount ofthe optical attenuator and the output optical power of the light source.

Meanwhile, in the first-fourth embodiments, the control data X and Yhaving the same data pattern as each other are input to the modulators10 and 20 when the power balance between the X polarization and the Ypolarization is adjusted. However, the control data X and Y does notneed to be the same as each other. For example, the data pattern of thecontrol data X may be the reversed phase data of the control data Y. Inthis case, the control data Y may be generated by reversing each symbolof the control data X.

FIG. 13 illustrates the input and output of the LN modulator in the casewhere the drive data are in reverse phase with each other. Here, theamplitude of the drive voltage of the LN modulator is supposed to be2Vπ. That is, the amplitude of the drive voltage of the modulators 10and 20 that operate as DQPSK modulators is 2Vπ. FIG. 13 illustrates theinput and output of one of the arms (I arm or Q arm) of the DQPSKmodulator.

In the LN modulators driven at 2Vπ, in the case where the drive signalsare in reserve phases as each other, the data patterns of the modulatedoptical signals output from the modulators become the same as eachother. In other words, in the modulators 10 and 20, when the controldata X and Y are the same patterns as each other, and when the controldata X and Y are in reverse phases as each other, the same modulatedoptical signals may be obtained.

Meanwhile, in the optical signal transmitter according to thefirst-fourth embodiments, when the power balance between the Xpolarization and the Y polarization is adjusted, the phases of the phaseshifters 11 and 21 are set to “A−Δφ” and “A+Δφ, respectively.Hereinafter, the control method of the phase shifters 11 and 21 isdescribed with reference to FIG. 14-FIG. 16. In FIG. 14-FIG. 16, formaking the drawings easier to view, the control system for controllingthe power balance is omitted.

For example, while there is no limitation, the phase shifter 11 and 21apply a voltage to one or both of a pair of optical paths provided ineach modulator and adjust the phase difference between the optical pathsby controlling the optical path length of the optical path. In examplesillustrated in FIG. 14-FIG. 16, the phase difference is adjustedaccording to the voltage of a phase shift control signal given from thecontroller.

Meanwhile, in the modulators 10 and 20 being DQPSK optical modulators,the AC component of the output optical power becomes local minimum whenthe phase difference between a pair of optical paths is zero, π/2, π, or3π/2. In other words, in the modulators 10 and 20, by controlling thevoltage to be applied to the phase shifters 11 and 21 so as to make theAC component of the output optical power local minimum, the phasedifference between the pair of optical paths converges at zero, π/2, π,or 3π/2. Here, it is assumed that the phase shift control voltage (Vs)to set the phase difference between the pair of optical pathsapproximately π/2 is obtained in advance based on the design of theoptical waveguide providing the optical paths of the phase modulators 10and 20.

In this case, when the phase shift control voltage is Vs, the ACcomponent of output optical power is local minimum. When the localminimum point of the AC component of the output optical power isdetected while the phase shift control voltage is gradually increasedfrom Vs, the phase difference between the pair of optical paths iseither zero or π. When the local minimum point of the AC component ofthe output optical power is detected while the phase shift controlvoltage is gradually decreased from Vs, the phase difference between thepair of optical paths is the other of zero and π.

In the optical signal transmitter according to the embodiment, forexample, the local minimum point described above is detected by thedithering method. In the configuration illustrated in FIG. 14, acontroller 91 generates a phase shift control signal. The phase shiftercontrol signal is, for example, a DC voltage. A low-frequency signalgenerator 92 generates a low-frequency signal. The frequency (f0) of thelow-frequency signal is sufficiently low with respect to the symbol rateof the data X and Y. A superimposer (adder) 93 superimposes (adds) thelow-frequency signal on the phase shift control signal. Therefore, thephase shift control signal on which the low-frequency signal has beensuperimposed is given to the phase shifters 11 and 21. Thus, the phaseprovided by the phase shifters 11 and 21 varies with the frequency ofthe low-frequency signal. As a result, the AC component of the outputoptical power of the modulators 10 and 20 also varies with the frequencyof the low-frequency signal.

Photo detectors (PD) 94X and 94Y respectively convert the output lightfrom the modulators 10 and 20. The photo detectors 94X and 94Y may beprovided within the modulators 10 and 20, or may be provided outside themodulators 10 and 20. A switch (selector) 95 selects an electric signalobtained by the photo detectors 94X or 94Y according to the instructionfrom the controller 91. A low pass filter 96 removes the high-frequencycomponent (for example, the symbol frequency) of the electric signalselected by the switch 95. An AC component extracting device 97 removesthe DC component of the electric signal. An AC component power detector98 detects the AC component power of the filtered electric signal. Atthis time, the output signal of the AC component power detector 98includes the f0 component.

The controller 91 generates the phase shift control signal using the f0component contained in the output signal of the AC component powerdetector 98 so that the AC component power becomes local minimum. Atthis time, when controlling the phase shifter 11 of the modulator 10,the switch 95 selects the electric signal of the photo detector 94X.Then, the controller 91 gives the generated phase shift control signalto the phase shifter 11. When controlling the phase shifter 21 of themodulator 20, the switch 95 selects the electric signal of the photodetector 94Y. Then, the controller 91 gives the generated phase shiftcontrol signal to the phase shifter 21.

In the configuration illustrated in FIG. 14, the low pass filter 96, theAC component extracting device 97, and the AC component power detector98 are shared for X polarization and Y polarization to control the phaseshifters 11 and 21. In contrast, in the configuration illustrated inFIG. 15, a low pass filter 96X, an AC component extracting device 97Xand an AC component power detector 98X are provided to control the phaseshifter 11, and a low pass filter 96Y, an AC component extracting device97Y and an AC component power detector 98Y are provided to control thephase shifter 21. Meanwhile, in the configuration illustrated in FIG. 14and FIG. 15, the control procedures are basically the same as those ofeach other.

The controller 91 illustrated in FIG. 14 and FIG. 15 may be realized asa part of the functions of the controller 55 in the first-fourthembodiments. In addition, the photo detectors provided for anotherpurpose (for example, the bias control of the modulators 10 and 20) maybe used as the photo detectors 94X and 94Y illustrated in FIG. 14 andFIG. 15.

In the configuration illustrated in FIG. 16, feedback control of thephase shifter 11 and 21 is performed using a polarization multiplexedoptical signal. In this configuration, the controller 55 generates thephase shift control signal. That is, the controller 55 may be equippedwith both the function of controlling the phase shifters 11 and 21, andthe function of controlling the power balance between the polarizations.

Next, the method of controlling the power balance between the Xpolarization and the Y polarization repeatedly or continuously duringthe period in which the optical signal transmitter is transmitting data.

In the embodiments described above, the power balance between the Xpolarization and the Y polarization is adjusted during the period inwhich the optical signal transmitter is not transmitting data. Theoptical signal transmitter starts the transmission of data after thepower balance between the X polarization and the Y polarization isadjusted. However, the power balance between the X polarization and theY polarization changes due to the temperature, aging and so on.Therefore, in the configuration described below, the power balancebetween the X polarization and the Y polarization is controlledregularly or continuously during data transmission, using the controlresult obtained in the method in the embodiments described above. In thefollowing description, the control performed during the period in whichthe optical signal transmitter is not transmitting data to a receivingstation may be called “offline control”, and the control performedduring the period in which the optical signal transmitter istransmitting data to a receiving station is called “online control”

FIG. 17 is a diagram illustrating the configuration of the opticalsignal transmitter that performs the online control. In this example,the configuration of the optical signal transmitter illustrated in FIG.17 is based on the first embodiment illustrated in FIG. 3. That is, theoptical signal transmitter has the photo detector 51, the AC componentextracting device 52, the low pass filter 53, the AC component powerdetector 54, and a controller 101. The controller 101 has the controlfunction provided by the controller 55 in the first embodiment. Theoptical signal transmitter that performs the online control may be basedon the configuration of the second-fourth embodiments.

The optical signal transmitter is capable of controlling the powerbalance between the X polarization and the Y polarization based on theoptical power of the modulated optical signals X and Y output from themodulators 10 and 20. That is, a photo detector 111, which is a photodiode for example, converts the modulated optical signal X generated bythe modulator 10 into an electric signal. Here, it is assumed that themodulator 10 outputs a pair of optical signals that are mutuallycomplementary. In this case, one of the pair of optical signals may beguided to the polarization beam combiner 31, and the other opticalsignals may be guided to the photo detector 111. Meanwhile, the photodetector 111 may be provided within the modulator 10. In addition,leaking light from a coupler at the output side of the modulator 10 maybe guided to the photo detector 111. Then, the DC component of theoutput signal of the photo detector 111 is given to the controller 101.The AC component of the output signal of the photo detector 111 is givento a modulator controller 112. The modulator controller 112 controls,for example, the bias of the modulator 10. In this case, the modulatorcontroller 112 operates as an ABC (Auto Bias Control) circuit.

A photo detector 121 converts the modulated optical signal Y generatedby the modulator 20 into an electric signal. Then, the DC component ofthe output signal of the photo detector 121 is given to the controller101. The AC component of the output signal of the photo detector 121 isgiven to the modulator controller 122.

As described above, the output signals of the photo detectors 111 and121 are given to the controller 101. Here, the output signals of thephoto detectors 111 and 122 correspond to the average output opticalpowers of the modulators 10 and 20, respectively. Therefore, thedifference between the output signals of the photo detectors 111 and 121may be a parameter indicating the power difference between the Xpolarization and the Y polarization. That is, by performing control tomake the difference between the output signals of the photo detectors111 and 121 zero, the power difference between the X polarization andthe Y polarization becomes small.

However, according to this method, the factor generated on the outputside of the adjusting point of the optical power (for example themodulators 10 and 20, the optical attenuators 13 and 23) cannot becompensated for. For example, when the characteristics of the photodetectors 111 and 121 are different from each other, even if thedifference between the output signals of the photo detectors 111 and 121is controlled to be zero, the power difference between the Xpolarization and the Y polarization contained in a polarizationmultiplexed optical signal does not become zero. In addition, thevariation in the combining characteristics of the polarization beamcombiner 31 may also cause a similar problem.

In order to solve this problem, the optical signal transmitterillustrated in FIG. 17 has a first control system (a control system thatuses the polarization multiplexed optical signal) and a second controlsystem (a control system that uses the modulated optical signals X andY). Then, the controller 101 performs the processes in the flowchart inFIG. 18.

Steps S1-S5 are offline control performed during the period in which theoptical signal transmitter is not transmitting data. In step S1, thepower balance between the X polarization and the Y polarization iscontrolled using the first control system. In step S1, as described withreference to FIG. 2 and FIG. 3, the power balance is controlled by thefollowing procedures.

(1) The phases of the phase shifters 11 and 21 are set to “A−Δφ” and“A+Δφ”, respectively. In DQPSK, A=π/2, and Δφ is, for example, 90degrees.(2) The control data X and Y having the same data pattern as each otherare given to the modulators 10 and 20.(3) One or more of the control signals C1-C3 are generated so as tominimize the monitor signal obtained by the AC component power detector54.

In step S2, the control result (init_adj_1) in S1 is stored in aspecified memory area. The control result to be stored is the settingvalue of power adjustment elements. The power adjustment elements are,in this example, the bias of the modulators 10 and 20, the amplitude ofdrive signal output from the drivers 12 and 22, and the attenuationamount of the optical attenuators 13 and 23. In addition, when the lightsources 2 and 3 are provided for the modulators 10 and 20 respectively,the output optical power of the light sources 2 and 3 is also one of thepower adjustment elements. Then, for example, when the power of the Xpolarization and/or the Y polarization is adjusted by controlling theattenuation amount of the optical attenuators 13 and 23, informationrepresenting the control voltage for controlling the attenuation amountof the optical attenuator is stored.

In step S3, the power balance between the X polarization and the Ypolarization is controlled using the second control system. In step S3,as described above, one or more of the control signals C1-C3 isgenerated so that the difference between the electric signals obtainedfrom the photo detectors 111 and 121 (that is, the powers of themodulated optical signal X and Y) becomes zero. At this time, the phasesof the phase shifters may be “A−Δφ” and “A+Δφ”, respectively, or may bethe optimal phase (that is, A) respectively. The data X and Y may be thesame as each other or may be different from each other. However, inorder to measure the error between the first control system and thesecond control system, it is preferable that the adjustment of the powerbalance is performed under the same conditions in step S1 and step S3.Meanwhile, it is assumed that in step S3, the same power adjustmentelement as in step S1 is adjusted. That is, when the optical attenuator13 and/or 23 is controlled in step S1, the optical attenuator 13 and/or23 is controlled in step S3.

In step S4, the control result (init_adj_2) in step S3 is stored in aspecified memory area. The control result to be stored is the settingvalue of the same power adjustment element as in step S2. Then, in stepS5, a correction value (offset) representing the error betweeninit_adj_1 stored in step S2 and init_adj_2 stored in step S4 iscalculated and stored in a specified memory area. Thus, in steps S1-S5,the error between the first control system and the second control systemis measured.

Steps S6-S7 are online control performed during the period in which theoptical signal transmitter is transmitting data. In step S6, thecorrection value (offset) stored in step S5 is set in the second controlsystem. In an example, the correction value (offset) is written into thememory area to which the controller 55 refers when the controllercalculates the control signals C1-C3.

In step S7, the second control system adjusts the power balance usingthe correction value (offset). At this time, the phase shifters 11 and21 are controlled to the optimal phase respectively. In addition, data Xand Y are data streams transmitted to a receiving station.

For example, it is assumed that “10” is obtained as init_adj_1, and“9.3” is obtained as init_adj_2 in the offline control. In this case,“0.7” is obtained as the correction value (offset). Then, the opticalsignal transmitter compensates for the error between the first controlsystem and the second control system using the correction value (offset)when the optical signal transmitter transmits data to the receivingstation. For example, when init_adj_2 obtained in the second controlsystem while data is being transmitted to the receiving station is“9.4”, the controller 55 outputs “10.1(=9.4+0.7)” for adjusting theoptical power adjustment element.

In the procedures described above, since the first control system is notsubject to the characteristics of the polarization beam combiner 31 orthe characteristics of the photo detectors 111 and 121, the accuracy ofthe control signal obtained in the first control system is high. In thecontrol method according to the embodiment, the measured value in thesecond control system obtained during the data transmission iscompensated for by the error between the first control system and thesecond control system. Therefore, according to the control method of theembodiment, the power difference between the X polarization and the Ypolarization may be constantly controlled to be small to obtain goodtransmission quality.

Other Embodiments

FIG. 19 is a diagram illustrating another configuration of the opticalsignal transmitter. The second control system of the optical signaltransmitter illustrated in FIG. 19 is, in this example, similar to theconfiguration illustrated in FIG. 17. However, the first control systemillustrated in FIG. 19 is different from the first control systemillustrated in FIG. 17.

The optical signal transmitter illustrated in FIG. 19 has a photodetector 131, a DC component detector 132, and a controller 133. Thephoto detector 131 converts a polarization multiplexed optical signalsplit by an optical splitter into an electric signal, in the same manneras the photo detector 51. The DC component detector 132 extracts the DCcomponent of the electric signal obtained by the photo detector 131. TheDC component is detected by, for example, integrating or averaging theinput signal from the photo detector 131. Then, the DC componentdetector 132 outputs the detection result as a monitor signalrepresenting the average power of the polarization multiplexed opticalsignal.

The controller 133 controls the power adjustment element (the bias ofthe modulators 10 and 20, the amplitude of the drive signal generated bythe drivers 12 and 22, the attenuation amount of the optical attenuators13 and 23, (the output optical power of the light sources 2 and 3)).Meanwhile, the controller 133 also provides the operation of the secondcontrol system.

FIG. 20 is a flowchart illustrating the operations of the controller 133illustrated in FIG. 19. Steps S11-S20 are performed in the offlinecontrol. In this embodiment, the phase of the phase shifters 11 and 21are maintained at the optimal phase in the offline control and theonline control. That is, for example, in DQPSK, the phase shifters 11and 21 are both maintained at π/2. In addition, data X and Y do not needto be the same as each other, and transmission data or arbitrary datapatterns are input.

In step S11, the modulated optical signal X (X polarization) iscontrolled to be ON and the modulated optical signal Y (Y polarization)is controlled to be OFF by controlling the power adjustment element.“ON” indicates a state in which light is emitted, and “OFF” indicates astate in which no light is emitted. When the power adjustment element isthe optical attenuators 13 and 23, for example, the attenuation amountof the optical attenuator 13 is controlled to minimum, and theattenuation amount of the optical attenuator 23 is controlled tomaximum. At this time, the modulated optical signal X is hardlyattenuated by the optical attenuator 13 and is guided to thepolarization beam combiner 31. On the other hand, the modulated opticalsignal Y is fully attenuated by the optical attenuator 23, thus is nottransmitted to the polarization beam combiner 31. Accordingly, thepolarization multiplexed optical signal output from the polarizationbeam combiner 31 substantially includes only the X polarization. In stepS12, the DC component DC_X detected by the DC component detector 132 isstored in a specified memory area. Here, since the polarizationmultiplexed optical signal substantially includes only the Xpolarization, the detected value (that is, DC_X) by the DC componentdetector 132 represents the power of the X polarization.

In step S13, the modulated optical signal X (X polarization) iscontrolled to be OFF, and the modulated optical signal Y (Ypolarization) is controlled to be ON. In the example described above,the attenuation amount of the optical attenuator 13 is controlled tomaximum, and the attenuation amount of the optical attenuator 23 iscontrolled to minimum. In step S14, the DC component DC_Y detected bythe DC component detector 132 is stored. Meanwhile, in steps S13-S14,the polarization multiplexed optical signal output from the polarizationbeam combiner 31 substantially includes only the Y polarization.Therefore, the detected value DC_Y represents the power of the Ypolarization.

In step S15, the power adjustment element is controlled so as to makethe difference between the DC component DC_X and the DC component DC_Ysmall. For example, when “DC_X−DC_Y>0”, control to increase theattenuation amount of the optical attenuator 13, and/or control todecrease the attenuation amount of the optical attenuator 23 isperformed. Note that the controller 133 stores the latest setting valuefor the power adjustment element, and the stored setting value isupdated in step S15.

In step S16, the difference obtained in step S15 and a specifiedthreshold value are compared. The threshold value is a sufficientlysmall value. That is, in steps S16, whether or not the differencebetween the DC component DC_X and the DC component DC_Y has converged atapproximately zero is determined. If the difference is larger than thethreshold value, the process returns to step S11. In other words, stepsS11-S15 are performed repeatedly until the difference becomes smallerthan the threshold value. Then, when the difference becomes smaller thanthe threshold value, the updated setting value init_adj_1 of the poweradjuster is stored in a specified memory area.

Next, in steps S18-S19, the power balance between the X polarization andthe Y polarization is controlled using the second control system. Theprocess is the same as steps S3-S4 illustrated in FIG. 18. That is, thesetting value init_adj_2 obtained in the second control system isstored.

In step S20, a correction value (offset) representing the error betweeninit_adj_1 stored in step S17 and init_adj_2 stored in step S19 iscalculated and stored in a specified memory area. Thus, in stepsS11-S20, the error between the first control system and the secondcontrol system is measured.

Steps S21-S22 are online control, which are basically the same as stepsS6-S7 illustrated in FIG. 18. That is, the correction value (offset)stored in step S20 is set in the second control system. Then, the secondcontrol system adjusts the power balance using the correction value(offset).

As described, in the configuration and method illustrated in FIG.19-FIG. 20, there is no need to set a special phase (A−Δφ and A+Δφ) forthe phase shifters to adjust the power balance between the Xpolarization and the Y polarization. That is to say, in thisconfiguration, the shifters 12 and 13 may be controlled to be optimalphase even in the offline control procedure. In addition, in theconfiguration and method illustrated in FIG. 19-FIG. 20, there is noneed to use particular data patterns as data X and Y to adjust the powerbalance between the X polarization and the Y polarization.

The offline control described above is performed, for example, at theinitial setting of the control system or when the wavelengths areswitched in the WDM system. The online control described above isrepeated regularly, for example. Alternatively, the online controldescribed above may be performed under a specified condition (forexample, when the temperature of the optical transmitter changes).

In addition, the optical signal transmitter to transmit QPSK (includingDQPSK) signal is described in the explanation above, the invention isnot limited to this configuration. The optical signal transmitteraccording to the invention may be configured to transmit a modulatedoptical signal of another modulation scheme.

Furthermore, there is no particular limitation on the configuration orsystem of the receiver that receives the polarization multiplexedoptical signal generated by the optical signal transmitter according tothe embodiments. The receiver may be, for example, a digital coherentreceiver, or may be a optical receiver using an interferometer.

All examples and conditional language recited herein are intended forpedagogical purposes to aid the reader in understanding the inventionand the concepts contributed by the inventor to furthering the art, andare to be construed as being without limitation to such specificallyrecited examples and conditions, nor does the organization of suchexamples in the specification relate to a showing of the superiority andinferiority of the invention. Although the embodiment (s) of the presentinventions has (have) been described in detail, it should be understoodthat the various changes, substitutions, and alterations could be madehereto without departing from the spirit and scope of the invention.

1. An optical signal transmitter comprising: a first outer modulator togenerate a first modulated optical signal, the first outer modulatorincluding a pair of optical paths and a first phase shifter to give aphase difference to the pair of optical paths; a second outer modulatorto generate a second modulated optical signal, the second outermodulator including a pair of optical paths and a second phase shifterto give a phase difference to the pair of optical paths; a combiner togenerate a polarization multiplexed optical signal by combining thefirst and second modulated optical signals; a phase controller tocontrol the phase difference by the first phase shifter to A−Δφ andcontrol the phase difference by the second phase shifter to A+Δφ whenthe first and second outer modulators are driven by first and secondcontrol data signals respectively; and a power controller to control atleast one of the first and second outer modulators based on an ACcomponent of the polarization multiplexed optical signal when the firstand second outer modulators are driven by the first and second controldata signals respectively; wherein a data pattern of the first controldata signal is same as the second control data signal or reversedpattern of the second control data signal.
 2. The optical signaltransmitter according to claim 1, wherein the power controller controlsat least one of the first and second outer modulators so as to make anaverage of the AC component of the polarization multiplexed opticalsignal small.
 3. The optical signal transmitter according to claim 1,wherein the first outer modulator includes a first driver circuit togenerate a drive signal for generating the first modulated opticalsignal; the second outer modulator includes a second driver circuit togenerate a drive signal for generating the second modulated opticalsignal; the first driver circuit generates a first drive signal from thefirst control data signal and the second driver circuit generates asecond drive signal from the second control data signal, when the firstand second control data signals are given to the first and second outermodulators, respectively; and the power controller controls an amplitudeof at least one of the first and second drive signals.
 4. The opticalsignal transmitter according to claim 1, wherein the first outermodulator includes a first inner modulator to generate the firstmodulated optical signal using electro-optic effect; the second outermodulator includes a second inner modulator to generate the secondmodulated optical signal using electro-optic effect; and the powercontroller controls a bias of at least one of the first and second innermodulators.
 5. The optical signal transmitter according to claim 1,wherein the first outer modulator includes a first optical attenuator;the second outer modulator includes a second optical attenuator; and thepower controller controls at least one of the first and secondattenuators.
 6. The optical signal transmitter according to claim 1,wherein the first outer modulator includes a first phase modulator and afirst intensity modulator serially connected to the first phasemodulator; the second outer modulator includes a second phase modulatorand a second intensity modulator serially connected to the second phasemodulator; each of the first and second intensity modulators modulatesan optical signal using electro-optic effect; and the power controllercontrols a bias of at least one of the first and second intensitymodulators.
 7. The optical signal transmitter according to claim 1,wherein the power controller gives a low-frequency signal to the firstand second outer modulators and controls at least one of the first andsecond outer modulators based on the low-frequency signal componentcontained in the polarization multiplexed optical signal.
 8. The opticalsignal transmitter according to claim 1, further comprising: a firstmonitor to obtain a first monitor value representing an optical power ofthe first modulated optical signal output from the first outermodulator; and a second monitor to obtain a second monitor valuerepresenting an optical power of the second modulated optical signaloutput from the second outer modulator, wherein the phase controllercontrols the phase difference by the first and second phase shifters toA when the first and second outer modulators are driven by first andsecond transmission data signals; and the power controller corrects acontrol signal controlling at least one of the first and second outermodulators so as to make the first and second monitor valuesapproximately same as each other, according to a control result based onthe AC component of the polarization multiplexed optical signal obtainedwhen the first and second control data signals are used, when the firstand second outer modulators are driven by the first and secondtransmission data signals.
 9. The optical signal transmitter accordingto claim 1, wherein A is an optimal phase determined according to anmodulation scheme of the first and second outer modulators.
 10. Anoptical signal transmitter comprising: a first light source; a firstouter modulator to generate a first modulated optical signal bymodulating an optical signal generated by the first light source, thefirst outer modulator including a pair of optical paths and a firstphase shifter to give a phase difference to the pair of optical paths; asecond light source; a second outer modulator to generate a secondmodulated optical signal by modulating an optical signal generated bythe second light source, the second outer modulator including a pair ofoptical paths and a second phase shifter to give a phase difference tothe pair of optical paths; a combiner to generate a polarizationmultiplexed optical signal by combining the first and second modulatedoptical signals; a phase controller to control the phase difference bythe first phase shifter to A−Δφ and control the phase difference by thesecond phase shifter to A+Δφ when the first and second outer modulatorsare driven by first and second control data signals respectively; and apower controller to control at least one of the first and second lightsources based on an AC component of the polarization multiplexed opticalsignal when the first and second outer modulators are driven by thefirst and second control data signals respectively; wherein a datapattern of the first control data signal is same as the second controldata signal or reversed pattern of the second control data signal.
 11. Amethod for controlling a polarization multiplexed optical signal in anoptical signal transmitter including a first outer modulator having apair of optical paths and a first phase shifter to give a phasedifference to the pair of optical paths to generate a first modulatedoptical signal; a second outer modulator having a pair of optical pathsand a second phase shifter to give a phase difference to the pair ofoptical paths to generate a second modulated optical signal; and acombiner to generate the polarization multiplexed optical signal bycombining the first and second modulated optical signals, comprising:controlling the phase difference by the first phase shifter to A−Δφ;controlling the phase difference by the second phase shifter to A+Δφ;generating first and second control data signals as drive signals of thefirst and second outer modulators, respectively; and controlling atleast one of the first and second outer modulators based on an ACcomponent of the polarization multiplexed optical signal generated whenthe first and second outer modulators are driven by the first and secondcontrol data signals, respectively, wherein a data pattern of the firstcontrol data signal is same as the second control data signal orreversed pattern of the second control data signal.
 12. A method forcontrolling a polarization multiplexed optical signal in an opticalsignal transmitter including a first outer modulator having a pair ofoptical paths and a first phase shifter to give a phase difference tothe pair of optical paths to generate a first modulated optical signal;a second outer modulator having a pair of optical paths and a secondphase shifter to give a phase difference to the pair of optical paths togenerate a second modulated optical signal; and a combiner to generatethe polarization multiplexed optical signal by combining the first andsecond modulated optical signals, comprising: in a first period,controlling the phase difference by the first phase shifter to A−Δφ;controlling the phase difference by the second phase shifter to A+Δφ;generating first and second control data signals as drive signals of thefirst and second outer modulators, respectively; generating a firstcontrol parameter for controlling at least one of the first and secondouter modulators so as to minimize an AC component of the polarizationmultiplexed optical signal; and generating a second control parameterfor controlling at least one of the first and second outer modulators soas to make optical powers of the first and second modulated opticalsignals approximately same as each other, wherein a data pattern of thefirst control data signal is same as the second control data signal orreversed pattern of the second control data signal, and in a secondperiod, controlling the phase difference by the first and second phaseshifters to A respectively, generating first and second transmissiondata as drive signal of the first and second outer modulators; andcorrecting the second control parameter based on an error between thefirst control parameter and the second control parameter, andcontrolling at least one of the first and second outer modulators by thecorrected second control parameter.
 13. A method for controlling apolarization multiplexed optical signal in an optical signal transmitterhaving a first outer modulator to generate a first modulated opticalsignal, a second outer modulator to generate a second modulated opticalsignal, and a combiner to generate a polarization multiplexed opticalsignal by combining the first and second modulated optical signals,comprising: in a first period, generating a first control parameter forcontrolling at least one of the first and second outer modulators so asto make a difference between an optical power of the polarizationmultiplexed optical signal obtained when the first outer modulator is inno emission state and an optical power of the polarization multiplexedoptical signal obtained when the second outer modulator is in noemission state approximately zero; generating a second control parameterfor controlling at least one of the first and second outer modulators soas to make an optical power of the first modulated optical signal and anoptical power of the second modulated optical signal approximately sameas each other; and in a second period, correcting the second controlparameter based on an error between the first control parameter and thesecond control parameter, and controlling at least one of the first andsecond outer modulators by the corrected second control parameter.